1) Field of the Invention
The invention relates to measuring a paper or board coating. Particularly the invention relates to measuring the coating by means of IR radiation.
2) Description of Related Art
Measuring a paper or board coating is an important part of the manufacturing process of paper or board, because an appropriate coating improves, for example, printability, gloss and color of paper. Instead of applying only one coating layer, the coating can be performed several times using several different materials. Coating materials include binders and coating pigments, and some of the coating components that are used include kaolin, calcium carbonate, talc, gypsum, latex, starch, many synthetic binders and special coatings, such as silicon.
A prior art coating system is described in U.S. Pat. No. 5,795,394, which is incorporated herein by reference. This coating system concentrates on measuring calcium carbonate and controlling the coating in the MIR region. In the solution of the publication, MIR radiation is directed at a coated substrate at two separate wavelength bands, one of which is sensitive to the substrate and the other is sensitive both to the substrate and to calcium carbonate. On the basis of these different wavelength band strengths, the amount of calcium carbonate is measured, and the amount of coating is adjusted by means of the measured calcium carbonate. As far as measurements of other components are concerned, the publication mentions the measurement of kaolin, and also the measurement of paper moisture. The problem of the solution given in the publication is that the amount of only a few components (mainly that of calcium carbonate) used in the coating can be measured, although a large number of coating materials can be used for coating paper or board. Since several detectors are used in the measurement, the equipment used is complicated and expensive. Another disadvantage of using several detectors is that the measurements are inaccurate, which is due to nonidealities in the optical path used by the detectors, which means that each detector sees the optics in a different manner. In addition, the strength of the signal to be detected weakens in proportion to the amount of components or detectors.
Another prior art solution to measure a coating is described in U.S. Pat. No. 5,338,361, which is incorporated herein by reference. In the solution according to the publication, components of two different coatings on the substrate are measured by using three separate NIR wavelength bands, the first wavelength band being sensitive to the first component, the second wavelength band to the second component and the third wavelength band to the substrate. In order to measure several different coatings more wavelength bands are used. Since this solution, too, employs several detectors in the measurement, the equipment used is complicated and expensive. A further disadvantage of using several detectors is that the measurements are inaccurate, which is due to nonidealities in the optical path used by the detectors, which means that each detector sees the optics in a different manner.
It is an object of the invention to implement an improved method and an apparatus implementing the method. This is achieved by a method for measuring a coating from paper or board by means of IR radiation, the coating comprising at least two components. The method further comprises the steps of carrying out the measurement by one detector; measuring at least one component from the coating by using MIR radiation and at least one component by using NIR radiation, and, in order to measure at least one component by using MIR radiation: directing IR radiation at the coating; chopping the IR radiation directed at the coating; bandpass filtering a MIR wavelength band of the component, which is sensitive to the absorption of said at least one component from the IR radiation emerging from the coating; measuring the strength of the MIR radiation sensitive to the absorption; bandpass filtering a MIR wavelength band of the component, which is insensitive to the absorption of the component from the IR radiation emerging from the coating; measuring the strength of the MIR radiation insensitive to the absorption; measuring the strength of the absorption of the component by comparing the strength of the MIR radiation sensitive to the absorption and the strength of the MIR radiation insensitive to the absorption with each other; determining the amount of at least one component of the coating on the basis of the measured strength of at least one MIR absorption; and, in order to measure at least one component by using NIR radiation: bandpass filtering a NIR wavelength band of the component, which is sensitive to the absorption of the component from the IR radiation emerging from the coating; measuring the strength of the NIR radiation sensitive to the absorption; bandpass filtering a NIR wavelength band of the component, which is insensitive to the absorption of the component from the IR radiation emerging from the coating; measuring the strength of the NIR radiation insensitive to the absorption; measuring the strength of the absorption of the component by comparing the strength of the NIR radiation sensitive to the absorption and the strength of the NIR radiation insensitive to the absorption with each other; determining the amount of at least one component of the coating on the basis of the measured strength of at least one NIR absorption.
The invention also relates to a method for measuring a coating from paper or board by means of IR radiation. The method further comprises the steps of simultaneously measuring from the coating at least one component by using MIR radiation and at least one component by using NIR radiation, and, in order to measure at least one component by using MIR radiation: directing IR radiation at the coating; chopping the IR radiation directed at the coating; bandpass filtering a MIR wavelength band of the component, which is sensitive to the absorption of said at least one component from the IR radiation emerging from the coating; measuring the strength of the MIR radiation sensitive to the absorption; bandpass filtering a MIR wavelength band of the component, which is insensitive to the absorption of the component from the IR radiation emerging from the coating; measuring the strength of the MIR radiation insensitive to the absorption; measuring the strength of the absorption of the component by comparing the strength of the MIR radiation sensitive to the absorption and the strength of the MIR radiation insensitive to the absorption with each other; determining the amount of at least one component of the coating on the basis of the measured strength of at least one MIR absorption; and, in order to measure at least one component by using NIR radiation: bandpass filtering a NIR wavelength band of the component, which is sensitive to the absorption of the component from the IR radiation emerging from the coating; measuring the strength of the NIR radiation sensitive to the absorption; bandpass filtering a NIR wavelength band of the component, which is insensitive to the absorption of the component from the IR radiation emerging from the coating; measuring the strength of the NIR radiation insensitive to the absorption; measuring the strength of the absorption of the component by comparing the strength of the NIR radiation sensitive to the absorption and the strength of the NIR radiation insensitive to the absorption with each other; determining the amount of at least one component of the coating on the basis of the measured strength of at least one NIR absorption.
The invention further relates to an apparatus for measuring a coating from paper or board by means of IR radiation, the coating comprising at least two components. The apparatus further comprises one detector for measuring at least one component from the coating by using MIR radiation and at least one component by using NIR radiation, the apparatus comprising: an optical power source for radiating IR radiation to the coating; a chopper for chopping the IR radiation directed at the coating; and, in order to perform MIR measurement for one component the apparatus comprises: a bandpass filter for filtering a MIR wavelength band of the component, which is sensitive to the absorption of the component from the IR radiation emerging from the coating; a detector is arranged to detect MIR radiation sensitive to the absorption and to convert the strength of the detected MIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the MIR radiation sensitive to the absorption; a bandpass filter for filtering a MIR wavelength band of the component, which is insensitive to the absorption of the component from the IR radiation emerging from the coating; the detector is arranged to detect MIR radiation insensitive to the absorption and to convert the strength of the detected MIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the MIR radiation insensitive to the absorption; the apparatus is arranged to measure the strength of the absorption of the component by comparing the strength of the MIR radiation sensitive to the absorption and the strength of the MIR radiation insensitive to the absorption with each other; the apparatus is arranged to determine the amount of at least one component of the coating on the basis of the measured strength of at least one MIR absorption; and, in order to perform NIR measurement for one component the apparatus comprises: a bandpass filter for filtering a wavelength band of at least one other component, which is sensitive to the absorption of said at least one other component in the NIR region from the IR radiation emerging from the coating; the detector is arranged to detect NIR radiation sensitive to the absorption and to convert the strength of the detected NIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the NIR radiation sensitive to the absorption; a bandpass filter for filtering a wavelength band of at least one other component, which is insensitive to the absorption of said at least one other component in the NIR region from the IR radiation emerging from the coating; the detector is arranged to detect NIR radiation insensitive to the absorption and convert the strength of the detected NIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the NIR radiation insensitive to the absorption; the apparatus is arranged to measure the strength of the absorption of the component by comparing the strength of the NIR radiation sensitive to the absorption and the strength of the NIR radiation insensitive to the absorption with each other; the apparatus is arranged to determine the amount of at least one component of the coating on the basis of the measured strength of at least one NIR absorption.
The invention further relates to an apparatus for measuring a coating from paper or board by means of IR radiation, the coating comprising at least two components. In addition, the apparatus is arranged to simultaneously measure at least one component from the coating by using MIR radiation and at least one component by using NIR radiation, the apparatus comprising: an optical power source for radiating IR radiation to the coating; a chopper for chopping the IR radiation directed at the coating; and, in order to perform MIR measurement for one component the apparatus comprises: a bandpass filter for filtering a MIR wavelength band of the component, which is sensitive to the absorption of the component from the IR radiation emerging from the coating; a first detector for detecting MIR radiation sensitive to the absorption and converting the strength of the detected MIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the MIR radiation sensitive to the absorption; a bandpass filter for filtering a MIR wavelength band of the component, which is insensitive to the absorption of the component from the IR radiation emerging from the coating; the first detector is arranged to detect MIR radiation insensitive to the absorption and to convert the strength of the detected MIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the MIR radiation insensitive to the absorption; the apparatus is arranged to measure the strength of the absorption of the component by comparing the strength of the MIR radiation sensitive to the absorption and the strength of the MIR radiation insensitive to the absorption with each other; the apparatus is arranged to determine the amount of at least one component of the coating on the basis of the measured strength of at least one MIR absorption; and, in order to perform NIR measurement for one component the apparatus comprises: a bandpass filter for filtering a wavelength band of at least one other component, which is sensitive to the absorption of said at least one other component in the NIR region from the IR radiation emerging from the coating; a second detector for detecting NIR radiation sensitive to the absorption and converting the strength of the detected NIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the NIR radiation sensitive to the absorption; a bandpass filter for filtering a wavelength band of at least one other component, which is insensitive to the absorption of said at least one other component in the NIR region from the IR radiation emerging from the coating; the second detector is arranged to detect NIR radiation insensitive to the absorption and convert the strength of the detected NIR radiation into an electrical signal of equal strength, by means of which signal the apparatus is arranged to measure the strength of the NIR radiation insensitive to the absorption; the apparatus is arranged to measure the strength of the absorption of the component by comparing the strength of the NIR radiation sensitive to the absorption and the strength of the NIR radiation insensitive to the absorption with each other; the apparatus is arranged to determine the amount of at least one component of the coating on the basis of the measured strength of at least NIR absorption.
The preferred embodiments of the invention are disclosed in the dependent claims.
The method and system of the invention provide a plurality of advantages. As the amount of components to be detected increases, the amount of detectors does not. Furthermore, the strength of the radiation to be detected does not decrease in proportion to the amount of components to be detected or to the amount of detectors. Interferences, for their part, are efficiently eliminated by combining radiation chopping and filtering.